Field of the Invention
The invention concerns a method for attenuation correction of emission tomography scan data, as well as a combined magnetic resonance emission tomography apparatus and a non-transitory, computer-readable data storage medium encoded with programming instructions, for implementing such a method.
Description of the Prior Art
In the scanner of a magnetic resonance apparatus, also called a magnetic resonance tomography system, the body of an examination person, in particular a patient, to be examined is conventionally exposed by a basic field magnet to a relatively high basic magnetic field, for example of 1.5 or 3 or 7 tesla. In addition, gradient switching operations occur with the use of a gradient coil unit. Radio-frequency pulses, for example excitation pulses, are then emitted by a radio-frequency antenna unit by suitable antenna devices, and this leads to the nuclear spins of specific atoms, which are excited in a resonant manner by these radio-frequency pulses, being tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. When the nuclear spins are relaxed, radio-frequency signals, known as magnetic resonance signals, are radiated from the atoms, which are received by suitable radio-frequency antennae and then processed further. Finally, the desired image data can be reconstructed from the raw data acquired in this way.
For a specific measurement, a specific magnetic resonance sequence, also called a pulse sequence, is therefore to be emitted, which includes a sequence of radio-frequency pulses, for example excitation pulses and refocusing pulses, and appropriate gradient switching operations that are to be emitted in a coordinated manner in various gradient axes in various spatial directions. At a time appropriate therewith readout windows are set, and these specify the periods in which the induced magnetic resonance signals are detected.
Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are imaging methods in nuclear medicine in which sectional images of an examination object are typically generated by scanning and rendering visible the distribution of a weakly radioactively marked substance in the body of the examination object using a radiation detector, in particular a specially designed PET detector or SPECT detector. Biochemical and physiological processes in an organ of the examination object can be depicted in this way. Before an examination, the examination object is injected with a radionuclide or a substance marked with a radionuclide.
The radionuclide emits positrons for a PET examination. When a positron interacts with an electron in the body of the examination object, two photons are emitted in opposing directions and the coincident detection thereof is registered using the PET detector of the PET apparatus. The spatial distribution of the radionuclide inside the body can be ascertained and sectional images of the inside of the body of the living organism can be generated from the registered random events.
To evaluate the results of an emission tomography scan, an attenuation map is required, which is a spatially resolved distribution of the attenuation values of the tissue of the current examination object. The attenuation values are typically stored in the form of linear attenuation coefficients with the unit 1/cm. The tissue of the examination object, which is located between the point of origin of the photons and the emission tomography detector, is especially relevant with respect to the attenuation correction. During the evaluation the emission tomography scan data are corrected with the use of the attenuation map. In the case of PET imaging the attenuation map includes, in particular, the attenuation values with respect to photons with an energy of 511 keV.
Medical examinations are often carried out by combined medical imaging devices that have more than one imaging modality, typically two imaging modalities. In these medical examinations diagnostic, scan data are acquired, in particular simultaneously, from an examination object by the multiple, in particular two, imaging modalities. The evaluation of the diagnostic image data reconstructed from the diagnostic scan data is thereby facilitated for a competent person, since the image data of the two imaging modalities are available to such a person. Combined magnetic resonance emission tomography devices, for example, are known. These include a combined magnetic resonance positron emission tomography device (magnetic resonance PET device) or a combined magnetic resonance single photon emission computed tomography device (magnetic resonance SPECT device).
The attenuation map for the attenuation correction of the emission tomography scan data is typically generated on the basis of magnetic resonance scan data. For example, magnetic resonance scan data can be used that have been acquired by execution of a gradient echo-based sequence. The advantage thereof is a short acquisition time, so that three dimensional magnetic resonance scan data for generating the attenuation map can be acquired during a breath-hold by the examination object in order to minimize movement artifacts due to breathing.
In magnetic resonance imaging, there is often an interference object in the examination object which disturbs the magnetic resonance imaging at least locally. An interference object of this kind can be, for example, metal. Possible interference objects are, for example, dental implants, dental braces, clips from operations (primarily in the thorax region), cardiac pacemakers, screws (primarily in the region of the spine) or joint replacement parts, such as, for example, knee implants or hip implants. The interference object can cause a magnetic interference field that results primarily from the different magnetic susceptibility of the metal contained in the interference object, compared to surrounding tissue. The magnetic interference field can lead to susceptibility artifacts that typically increase with the strength of the basic magnetic field of the magnetic resonance device.
This interference field, in particular the susceptibility artifacts, can lead to signal cancellations in the magnetic resonance scan data. It is also conceivable for the interference field to lead to signal enhancements. In general the interference field can lead to signal changes which shall hereinafter include signal cancellations and/or signal enhancements. A suitable attenuation value for these signal changes typically cannot be ascertained from the magnetic resonance scan data during generation of the attenuation map. This can lead to locally incorrect emission tomography image data that have been attenuation corrected by an attenuation map of this kind.